Atherosclerosis is a major health problem with an annual mortality of 500,000 deaths in the United States. It is currently accepted that acute coronary syndromes are most commonly the result of disruption of atheromatous vulnerable plaques that are angiographically modest in severity.
“Vulnerable plaque” is used to refer to a subgroup of only modestly stenotic but unstable plaques that are prone to rupture and, as a result, cause sudden cardiac arrest. While coronary angiography is widely used to illustrate and monitor luminal narrowing of the coronary artery, it is unable to provide selective identification of vulnerable plaques. It is known that approximately one-half of the unstable coronary atherosclerotic plaques are in arteries with 50% or less luminal diameter narrowing. These are lesions that are usually considered insignificant anatomically. Thus, it would be highly desirable if methods and devices were available to detect the unstable atherosclerotic plaque, independent of the degree of luminal diameter narrowing, and treat it before unstable angina and/or acute myocardial infarction and their consequences occur.
In general, atherosclerotic plaque at high risk for rupture contains large lipid pool(s) covered with a thin fibrous cap with ongoing inflammation and neovascularity. Nakamura, M., et al., Identification and treatment of vulnerable plaque. Rev Cardiovasc Med. 2004; 5 Suppl 2:S22-33. Recent reviews of features of vulnerable plaques (Naghavi, M., et al., From Vulnerable Plaque to Vulnerable Patient. A Call for New Definitions and Risk Assessment Strategies: Part I, Circulation. 2003; 108:1664-72) have emphasized the interactions of features of the plaque with systemic characteristics (Naghavi, M., et al., From vulnerable plaque to vulnerable patient: a call for new definitions and risk assessment strategies: Part II. Circulation. 2003 Oct. 14; 108(15): 1772-8).
Most of the alternative approaches to identify vulnerable plaques are based on often risky invasive endovascular approaches. Therefore, the development of non-invasive technology which enables vulnerable plaques to be distinguished from stable ones, is critical and urgently needed to reduce the morbidity and mortality of atherosclerotic patients.
Molecular approaches have been used to the problem of detecting and treating vulnerable plaques. Phage display has been used to identify peptide motifs that home to specific vascular beds (Arap, W., et al., Steps toward mapping the human vasculature by phage display, Nat. Med. 2002 February; 8(2):121-7), tumor lymphatics (Laakkonen, P., et al., A tumor-homing peptide with a targeting specificity related to lymphatic vessels, Nat Med. 2002 July; 8(7):751-5. Epub 2002 Jun 10) and a endothelial cell-specific LOX-1 receptor (White, S. J., et al., Identification of peptides that target the endothelial cell-specific LOX-1 receptor, Hypertension. 2001 February; 37(2 Part 2):449-55). See also Johns, M., et al., In vivo selection of sFv from phage display libraries, J Immunol Methods. 2000 May 26; 239(1-2):137-51 and Litovsky, S., et al., Superparamagnetic iron oxide-based method for quantifying recruitment of monocytes to mouse atherosclerotic lesions in vivo: enhancement by tissue necrosis factor-alpha, interleukin-1 beta, and interferon-gamma, Circulation. 2003 Mar. 25; 107(11):1545-9. In other studies, genes differentially expressed in plaques have been studied using suppression substractive hybridization (Faber, B. C., et al., Identification of genes potentially involved in rupture of human atherosclerotic plaques, Circ Res. 2001 Sep. 14; 89(6):547-54) and hybridization to oligonucleotide microarrays (Archacki, S. R., et al., Identification of new genes differentially expressed in coronary artery disease by expression profiling, Physiol Genomics. 2003 Sep. 29; 15(1):65-74).
An effective imaging approach for detection of vulnerable plaque should be based on the underlying biology. Knowing what lies within a plaque is a way to anticipate and prevent future events. A marker that can indicate the composition of the lesion is needed to predict the risk of plaque rupture. Myeloperoxidase, an enzyme released by activated macrophages may be one of those markers.
Myeloperoxidase is a heme containing enzyme, composed two 55 kDa subunits and two 15 kDa subunit, that uses H2O2 as a substrate to generate products that oxidize lipids and proteins. One such product, hypochlorous acid (HOCl), is critical in host defense against bacteria, viruses and tumor cells, and can also cause injury to normal tissue such as vascular epithelium. There is evidence that myeloperoxidase is generated and released by macrophages in plaque, and is believed to contribute to atherogenesis by catalyzing oxidative reactions (Daugherty, A., et al., Myeloperoxidase, a catalyst for lipoprotein oxidation, is expressed in human atherosclerotic lesions. J Clin Invest 1994; 94:437-444). Plaque rupture is consistent with a thin fibrous cap as well as high macrophage content, and these macrophages are known to secrete myeloperoxidase extracellularly in response to activation (Klebanoff, S. F., Oxygen metabolism and the toxic properties of phagocytes. Ann Int Med. 1980; 93:480-489). Immunohistochemistry has demonstrated an increased number of myeloperoxidase-expressing macrophages in eroded or ruptured plaques (Sugiyama, S., et al., Macrophage myeloperoxidase regulation by granulocyte macrophage colony-stimulating factor in human atherosclerosis and implications in acute coronary syndromes. Am J Pathol 2001; 158:879-891), with myeloperoxidase and macrophages found to co-localize in sections of atherogenic lesions. There has also been shown to be widespread immunostaining of myeloperoxidase consistent with both intra- and extracellular distributions of the enzyme in macrophage rich areas, and intense foci of immunostaining for myeloperoxidase in lipid rich regions of advanced atherosclerotic lesions. Myeloperoxidase oxidation of lipoproteins is believed to increase the negative charge thus forming products that stimulate cholesterol deposition in macrophages (Hazfil, L. J., & Stocker, R., Oxidation of low-density lipoprotein with hypochlorite causes transformation of the lipoprotein into a high-uptake form for macrophages. Biochem J. 1993; 290:165-172).
Myeloperoxidase generated HOCl was also found to promote selective oxidative cleavage of plasmalogens, liberating chloro fatty aldehydes and unsaturated lysophosphatidylcholine in human atherosclerotic lesions (Thukkani, A. K., et al., Identification of alpha-chloro fatty aldehydes and unsaturated lysophosphatidylcholine molecular species in human atherosclerotic lesions. Circulation 2003; 108: 3128-3133). Myeloperoxidase is believed to play a critical role in the development of atherosclerotic lesions by augmenting oxidative stress. Myeloperoxidase, as a component of atherosclerotic lesions, is a marker that can indicate the composition of the lesion as a marker of vulnerable plaques.